Work, energy, and power

definitions of these and other terms related to kinetics

WORK involves the application of a force over a distance.

Therefore, we measure work in terms of a:

Because ENERGY (E) is the capacity to do WORK, we measure energy and work in the same units (foot*lbs. or N*m).

E=Fd for "straight-line" work

E=Mr for work around an axis of rotation

POWER (P) is the rate of energy generation (or absorption) over time:

P = E/t

Power's SI unit of measurement is the Watt, representing the generation or absorption of energy at the rate of 1 Joule/sec. Power's unit of measurement in the English system is the horsepower, which is equivalent to 735.7 Watts.

For translational motions:

for rotational motions:

P = Fd/t

P = Mr/t

If, in the equation for rotational motion, which includes motion around a joint axis, we substitute:

for angular velocity (measured in degrees or radians per second), we obtain the familiar equation for JOINT POWER:


so that JOINT POWER is the product of:


Last updated 4-22-01 ©Dave Thompson PT